1. Field of the Invention
The present invention relates to a high strength aluminum alloy extruded material excellent in stress corrosion cracking resistance, and relates specifically to an aluminum alloy extruded material suitably used as structural materials for automobiles such as a bumper reinforce, door guard bar, and the like.
2. Description of the Related Art
In order to reduce the weight of an automobile, Al—Zn—Mg-based high strength aluminum alloy extruded materials (refer to Japanese Unexamined Patent Application No. 2002-327229, Japanese Unexamined Patent Application No. H11-264044) are used as energy absorption members such as a bumper reinforce, door guard bar, and the like. However, the Al—Zn—Mg-based aluminum alloy extruded material has a risk of causing stress corrosion cracking (hereinafter referred to as SCC), is obligedly subjected to an overaging treatment and is used at the proof stress of approximately 300 N/mm2 in order to avoid the risk, and is weakened in features as a high strength alloy.
For the purpose of further reducing the weight of an automobile, high-strengthening is required in the Al—Zn—Mg-based aluminum alloy extruded material used for the structural materials for automobiles such as a bumper reinforce and the like. However, when Zn and Mg are highly contained in order to achieve high-strengthening of the present Al—Zn—Mg-based alloy, the SCC resistance deteriorates because MgZn2 of intergranular precipitates is distributed in a high density, and the alloy cannot be applied as the structural materials for automobiles. Also, the extrusion performance deteriorates, thinly formation becomes hard, and, as a result, the weight reducing effect cannot be exerted.